The development of transistors employing organic materials (for instance, organic semiconductor materials) that exhibit semi-conducting electrical conductivity has recently progressed. These transistors have advantages such as being thinner, lighter, having lower material costs, and so forth, supporting their use as a switching element of a flexible display and so forth. Transistors have been proposed in which a source electrode and a drain electrode are formed on a substrate while an organic semiconductor layer, a gate insulating layer and a gate electrode are deposited above these electrodes in that order. These transistors can be fabricated in the atmosphere by using vapor deposition or a coating method.
However, the fact that these transistors can be fabricated in an atmosphere does not necessarily lead to the stable operation thereof in the atmosphere. Specifically, in the atmosphere, the organic semiconductor layer is doped with oxygen and water, which increases an off-state current, or in contrast increases traps so as to cause the deterioration of subthreshold swing (S value) and threshold voltage (Vth). A method of encapsulating a transistor has been proposed as a method of preventing an organic semiconductor layer from being doped with oxygen and water in the atmosphere.
Specifically, a transistor is fabricated on a glass substrate and then an encapsulating film is deposited in vacuum so as to cover the transistor, thereby encapsulating it. This prevents the organic semiconductor layer from being doped with oxygen and water, and thus the characteristics of the transistor can be stabilized. Also, a method has been proposed in which a multilayer of inorganic films and polymer films alternately deposited is used as an encapsulating film for further enhancing the encapsulating characteristic.
However, the methods of encapsulating a transistor result in an increased cost for the transistor because vacuum treatment and heat treatment are required. In addition, in the case of using a plastic substrate as a substrate, a gas barrier film also needs to be formed on the plastic substrate side because of the low gas-barrier characteristic of plastic materials, thereby further increasing costs.
Also, if a non-flexible component material (for example, silicon oxide or silicon nitride) is used as an encapsulating film, the flexibility of a transistor may be reduced. Even if these films are sufficiently thin, cracks tend to be generated in the films when a substrate is bent, or temperature or humidity varies. It, therefore, is difficult to achieve high reliability. Accordingly, a technique in which a gate electrode is composed of a metal film formed by vapor deposition has been proposed, for example, as a method of encapsulating a transistor at lower costs. Since a metal film formed by vapor deposition is a dense film and therefore exhibits a comparatively high barrier characteristic against water and oxygen, forming the metal film as a gate electrode to cover a channel part enables the metal film to function as an encapsulating material.
However, although the transistor thus fabricated exerts a comparatively high encapsulating characteristic, it is impossible to obtain a complete encapsulating. Therefore, if a transistor is subjected to a high temperature and high humidity environment and oxygen and water enter the inside thereof once, the high encapsulating characteristic functions adversely. That is, even when the transistor is brought back to a low temperature and low humidity environment, the oxygen and water are not discharged outside the transistor but remain in the inside thereof for a long time. Oxygen and water trapped therein deteriorate the characteristics of the transistor.
The following is a list of relevant documents related to the application: U.S. Pat. Nos. 6,150,191, 5,574,291, International Patent Publication No. 0147043, International Patent Publication No. 0147045, and Yong Qiu et al. (five coauthors), “H2O effect on the stability of organic thin-film field-effect transistors”, APPLIED PHYSICS LETTERS, Unites States, American Institute of Physics, 2003 Aug. 25, Vol. 83, No. 8, p. 1644-1646.